Method for producing a solid electrolyte cell

ABSTRACT

A method for producing a solid electrolyte cell employing a poly(ethylene oxide) plus EC/PC-containing cathode; a poly(ethylene oxide) plus EC/PC-containing solid electrolyte separator and an anode strip, such as lithium, sandwiched between current collector sheets, such as cooper sheets, that are sealed at their peripheral area by an adhesive coated frame.

FIELD OF THE INVENTION

The invention relates to a method for producing a solid electrolytecell, specifically a flat solid electrolyte cell.

BACKGROUND OF THE INVENTION

Ionic conductivity is commonly associated with the flow of ions througha liquid solution of salts. In the vast majority of practical uses ofionic conductors, i.e., as electrolytes for dry cell and sealed leadacid batteries, the liquid solution is immobilized in the form of apaste or gelled matrix or is absorbed in a separator to overcome thedifficulties associated with handling and packaging a liquid. However,even after immobilization, the system is still subject to possibleleakage, has a limited shelf life due to drying out or crystallizationof the salts and is suitable for use only within a limited temperaturerange corresponding to the liquid range of the electrolyte. In addition,the use of a large volume of immobilizing material has hindered the aimsof miniaturization and lowers the output capacity.

Improved microelectronic circuit designs have generally decreased thecurrent requirements for electronic devices. This in turn has enhancedthe applicability of solid electrolyte power sources which usually candeliver currents only in the microampere range. These solid electrolytesystems have the inherent advantages of being free of electrolyteleakage, corrosion and internal gassing problems due to the absence of aliquid phase. In addition, they also have a much longer shelf life thanthe conventional liquid electrolyte power sources.

In attempting to avoid the shortcomings of liquid systems, investigatorshave surveyed a large number of solid compounds seeking to findcompounds which are solid at room temperature and have specificconductances approaching those exhibited by the commonly used liquidsystems. Solid electrolytes must be essentially electronic insulators soas not to internally short the cell while at the same time they mustallow for ionic migration if the cell is to operate properly. There aremany solid state electrolytes "disclosed" in the art that can be usedfor solid state cells but many can only operate efficiently at highertemperatures, have low operating voltages or have internal highresistance.

It is an object of the present invention to provide a method forassembling a solid electrolyte cell, specifically a flat solidelectrolyte cell.

It is another object of the present invention to provide a method forassembling a solid electrolyte cell employing a solid electrolyte filmcontaining poly(ethylene oxide) or a poly(ethylene oxide) type polymerin conjunction with ethylene carbonate and propylene carbonate.

It is another object of the present invention to provide a method forassembling a solid electrolyte cell employing an active cathode filmcontaining poly(ethylene oxide) in conjunction with ethylene carbonateand propylene carbonate.

The foregoing and additional objects will become more fully apparentfrom the following description and drawing.

SUMMARY OF THE INVENTION

The invention relates to a method for producing a solid electrolyte cellcomprising the steps (a) depositing an active cathode materialcontaining ethylene carbonate and propylene carbonate onto a firstconductive current collector sheet and within an adhesive coated framepositioned at the peripheral area of the current collector sheet; (b)depositing on the cathode material propylene carbonate or ethylenecarbonate and propylene carbonate and allowing said solution to beabsorbed into the cathode material; (c) placing a separator strip ontothe cathode material, said separator strip being smaller than the areaof the cathode collector sheet and larger than the exposed area of thecathode material thereby leaving a portion of the adhesive coated framearea exposed; (d) placing an anode strip onto the separator and withinthe frame thereby leaving an exposed portion of the area of the adhesivecoated frame; (e) placing a second conductive current collectorsubstantially the size of the first conductive current collector ontothe anode and exposed area of the adhesive coated frame; and (f) heatingthe cell assembly at a pressure and temperature for a period of timesufficient to secure the first current collector to the adhesive coatedframe on one side and the opposite side of the adhesive coated frame tothe second current collector thereby producing a sealed cell.

As used herein, the word strip means a film, foil or a composite that isspread out to produce a film on a substrate.

The solid electrolyte strip could be fabricated from a composition ofpoly(ethylene oxide), referred to hereinafter as PEO, along with alithium salt, the anion of which may, for example, be I-, Br-, ClO₄ -,SCN-, BF₄ -, PF₆ - or CF₃ SO₃ -. Added to this composition is ethylenecarbonate and propylene carbonate. It has been found that ethylenecarbonate is better than propylene carbonate as an electrolyte solventbecause it has a higher electric constant, but has the disadvantage, foruse in a liquid system, that it is solid at room temperature. Thus, forsolid electrolyte applications, ethylene carbonate would be the desiredchoice. However, it was discovered in copending application Ser. No.421,085 filed Oct. 13, 1989 that the addition of propylene carbonatealong with ethylene carbonate to a poly(ethylene oxide)-containing solidelectrolyte will effectively lower the temperature at which the polymerundergoes a transition from an amorphous form to a crystalline formthereby substantially eliminating the presence of a crystalline form ofthe polymer at temperatures above about 20° C. This composition of asolid electrolyte is excellent for use in a solid electrolyte cell thatcan function at temperatures of about 20° C. and above.

The polymeric solid electrolyte film for use in this invention functionsas a physical barrier between the anode and the cathode material, aswell as, being ionically conductive at temperatures of 20° C. Thepreferred composition of the solid electrolyte separator would bePEO-70wt/%(3EC-1PC)₂₀ LiClO₄. The preferred preparation of the polymericsolid electrolyte would be as follows:

A desired quantity of ethylene carbonate is dissolved with propylenecarbonate in a small beaker. The beaker is covered and set aside untilthe ethylene carbonate is dissolved completely. The beaker may be heatedslightly (50° C.) to expedite the process. Dried poly(ethylene oxide) iscombined in a high density polyethylene bottle containing 3/4 inchdiameter ceramic mixing balls with isopropyl alcohol. The solution alongwith a metal salt, ethylene carbonate, propylene carbonate, and asolvent can then be ball milled for a time period such as 30-45 minutesuntil a smooth viscous mixture is formed.

The polymeric electrolyte solution can then be cast onto a release papersuch as a polyethylene coated release paper. The film is then allowed todry for example about 2 hours. The film can then be transferred into acontrolled temperature and humidity atmosphere (dry room) to completethe drying cycle. The material should have a moisture content less thanabout 30, preferably less than about 20 ppm H₂ O for battery use. Highermoisture levels result in a tacky film with poor mechanical properties.In addition, a latent reaction between the water and the salt (forexample LiClO₄), the water and the lithium and/or the water and thesolvent may also occur in a sealed cell if the water content is toohigh.

The molecular weight of the PEO can vary from 600,000 to 5,000,000. Theproportions of the EC to PC could vary between 3.4 to 0.5 and 0.5 to3.5. The amount of the PEO component of the solid electrolyte could varyfrom 30 to 50 weight percent. Suitable solvents for use in preparing thesolid electrolyte could be acetonitrile, methanol, tetrahydrofurane(THF), isopropyl alcohol, dichloromethane and the like.

The cathode material for use in this invention can contain an activecathode material such as manganese dioxide (MnO₂), carbon monofluoride,vanadium pentoxide, metal chromate such as silver chromate and silverbismuth chromate and silver vanadium chromate; metal oxide such asnickel oxide, lead oxide, bismuth lead oxide and copper oxides; sulfidessuch as copper sulfides and iron sulfides; and cadmium. A carbonaceousmaterial, if used, should preferably be carbon. The preferredcarbonaceous material is acetylene or furnace black. The cathodematerial should also contain the same material as the electrolyte suchas poly(ethylene oxide) with a lithium salt, the anion of which may, forexample, be I-, Br-, C1O₄ -, SCN-, BF₄ -, PF₆ - or CF₃ SO₃ -, along withethylene carbonate and propylene carbonate. The solvent for the cathodecould be methanol, trichloroethylene and the like. The preferredpreparation of the cathode material would be the following.

A quantity of ethylene carbonate can be dissolved with propylenecarbonate in a small beaker. The container could then be covered and setaside until the ethylene carbonate is completely dissolved. The beakermay be heated slightly (50° C.) to expedite the process. Pre-treatedmanganese dioxide and carbon could be mixed in their dry states in ahigh density polyethylene bottle with 3/4 inch diameter ceramic mixingballs for one hour. Upon completion of the dry blend, a solvent such asmethanol can be added. The mix can then be milled for about 1 hour. Asecond quantity of a solvent such as methanol and dried poly(ethyleneoxide) can then be added slowly, alternating between small additions ofliquid and dry materials, shaking vigorously between each combination.Next a salt such as a LiClO₄ salt can be added and the compositionshaken once again. Finally, a second solvent, such as trichloroethylene,the dissolved EC/PC solution and a dispersant such as sorbitanmonoleate, can be blended into the previous manganese dioxide-containingmixture and then can be milled for one hour. The composite can then bedegassed and cast onto a coated release paper substrate such aspolyethylene or directly onto a conductive current collector. The filmcan be allowed to dry for about 2 hours. The film can be transferred ina controlled temperature and humidity atmosphere (dry room) to completethe drying cycle. The material should have a moisture content less thanabout 30, preferably less than about 20 ppm H₂ O for cell use. Highermoisture levels result in a tacky film with poor mechanical properties.In addition, a latent reaction between the water and LiClO₄, the waterand the lithium and/or the water and the solvent may also occur in asealed cell if the water content is too high.

Additional EC and PC should be added to the cathode material prior toits assembly into a cell to replace any of the EC/PC that may have beenlost during the drying step. Also additional EC/PC should be added tofacilitate the proper orientation and location of the separator in theflat cell construction since the EC/PC will effectively prevent theseparator from shifting since EC/PC will make the cathode materialsomewhat tacky.

The current collector for use in this invention could be copper, nickel,stainless steel or the like, with copper being the preferred currentcollector and more preferably the copper could be surface treated toenhance its affinity for adhesion to other materials such as the frame.For example, the copper could be electrodeposited so that the surfacewould be roughened. Preferably the thickness of the current collectorfor most applications could be from 0.0005 to 0.003 inch thick. Theadhesive coated frame could be made of a plastic material such aspolyester, polyethylene or the like. Any suitable adhesive can beemployed as long as it can seal the current collectors together andprovide at least a liquid tight seal. Some examples of an adhesive areethylene vinyl acetate (EVA), polyethylene, ethylene acrylic acid (EAA),with EVA being the preferred adhesive.

The assembled cell should be heated under vacuum and pressure at theframe area to insure that the adhesive will firmly secure the currentcollectors to the frame. For most applications, the frame area of thecell assembly could be heated for about 125 to 200° C. under vacuum ofat least 20 inches and at a pressure between 40 and 100 psig for aslittle as 3 seconds. Preferably, the cell assembly could be heatedbetween about 170 to 175° C. at a pressure between 75 to 80 psig toproperly seal the cell. If desired, the adhesive frame could be firmlysecured to the first current collector and then the components of thecell could be assembled onto the current collector. The second currentcollector could be appropriately sealed to the frame during the finalsealing procedure. Also, if desired, an adhesive could be deposited ononly one side of the frame and then the adhesive coated side could besealed to the first current collector. Thereafter, the same or differentadhesive could be deposited on the exposed area of the frame and thensecured to the second current collector.

The solid electrolyte cell may be encapsulated in various laminates toprovide additional protection for the cell. However, if the cell isencapsulated in a film such as a polyamide or metalized polyethylenefilm, then provisions should be made so that electrical contact can bemade from outside the cell to the current collectors of the cell. Thiscould be accomplished by providing an opening in the film therebyexposing a selected area of each of the current collectors.

The present invention will become more apparent from the followingdescription thereof when considered therein with the accompanyingdrawing which is set forth as being exemplary of an embodiment of thepresent invention and is not intended in any way to be limitativethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded assembly view of a procedure for assembling asolid electrolyte flat cell of this invention.

FIG. 2 is a cross-sectional view of a solid electrolyte flat cellproduced using the method of this invention as described in conjunctionwith FIG. 1.

FIG. 1 shows a first current collector plate 2 onto which is placed acathode material 4. As shown, the surface area of the cathode material 4is smaller than the surface area of current collector 2 thereby leavinga peripheral exposed area 6. A frame 8, having an adhesive coating onboth sides, is disposed around cathode material 2 and contacts area 6 ofcurrent collector 2. If desired, the frame could be secured to thecurrent collector by a heat treatment using impulse heating, ultrasonicheating or the like. To compensate for any loss of the EC/PC componentin the cathode material 4, a suitable amount of EC/PC, preferably indroplet form 10, is deposited on top of cathode material 4. After theEC/PC solution 10 has been substantially absorbed into the cathodematerial 4, a solid electrolyte separator 12 is placed over cathodematerial 4. As shown in FIG. 1, separator 12 has a surface area largerthan the surface area of cathode material 4 but smaller than the exposedarea of frame 8 thus leaving an exposed area 14 of frame 8. An anodestrip 16 such as a lithium foil, is then placed on top of separator 12.As shown in FIG. 1, lithium foil 16 has a surface area smaller than thesurface area of separator 12. A top or second current collector 18 isplaced over the lithium foil 16 and contacts the exposed peripheral area14 of frame 8. This completes the assembly of a flat solid electrolytecell 20 employing a PEO-EC/PC-containing solid electrolyte separator 12and a PEO-EC/PC-containing cathode material 4.

FIG. 2 shows a cross-sectional view of the solid electrolyte cell 20produced as described in conjunction with FIG. 1 and has identicalcomponents identified with the same reference numbers. Specifically,FIG. 2 shows a solid electrolyte cell 20 comprising a laminatedstructure of a cathode material 4, solid electrolyte separator 12 andanode strip 16 disposed between a first current collector 2 and a secondcurrent collector 18. The current collectors 2 and 18 are sealed attheir peripheral areas by an adhesive coated frame 8. The overallassembled cell produced can be used to operate any electrical devicedesigned to operate at a cell output potential. Although not shown, thecell could be encased in an enclosure such as a plastic enclosure havingappropriate openings so that electrical contact could be made to bothcurrent collectors. The flat cell so produced occupies only a relativelysmall space and therefore can accommodate a variety of small batteryoperated devices. The cell can be fabricated with various outputcapacities and sizes to accommodate various size electrical devices.

EXAMPLE

A sample cell was made using the procedure as described in FIG. 1. Thecathode material was made from a composition as follows:

40.0 grams of heat treated manganese dioxide

2.28 grams of heat treated carbon

23.36 grams of ethylene carbonate dissolved with 9.04 grams of propylenecarbonate

4.0 grams of LiClO₄ salt

15.56 grams of poly(ethylene oxide) (PEO)

240 ml of methanol

320 ml of trichloroethylene of SPAN 80 which is a trademark for sorbitanmonooleate of ICI, Atlas Chemical Division of United States

The cathode material was made as described above and attached onto acopper current collector sheet using heat and pressure means. Anadhesive frame was placed onto the copper current collector in which thecathode material was positioned within the frame.

Second drops of an ethylene carbonate and propylene carbonate mixturewere applied to the exposed surface of the deposited manganese dioxideelectrode contained within the adhesive frame. A thin piece of a solidelectrolyte (separator) was placed on top of the manganese dioxideelectrode within the adhesive frame. The composition of the solidelectrolyte was:

21.60 grams of a complex of poly(ethylene oxide)

5.16 grams of a lithium salt, LiClO₄

37.80 grams of ethylene carbonate

12.60 grams of propylene carbonate

75 ml of isopropyl alcohol

1.5 mil thick piece of lithium was placed on top of the solidelectrolyte, followed by a second sheet of copper approximately the samesize as the copper current collector. The cell was placed under 80 psigpressure and heated to a temperature of about 175° C. so that theperipheral adhesive frame was sealed to the copper current collectorthereby providing a sealed Li/MnO₂ cell. The cell was discharged acrossa 33K ohm load and the voltage observed with time is shown in Table 1.The open circuit voltage was 3.45 volts.

                  TABLE 1                                                         ______________________________________                                        Li/MnO.sub.2 Cell Discharge                                                          Time    Voltage                                                               (hours) (volts)                                                        ______________________________________                                                0 (CCV)                                                                              3.392                                                                  1      3.208                                                                  2      3.134                                                                  3      3.077                                                                  4      3.042                                                                  5      3.016                                                                  6      2.995                                                                  7      2.978                                                                  8      2.966                                                                  9      2.952                                                                  10     2.929                                                                  15     2.899                                                                  20     2.755                                                                  50     2.737                                                                 100     2.737                                                                 150     2.561                                                                 240     1.843                                                          ______________________________________                                    

Two cells were connected in series in which one of the copper collectorswas removed so that the cell formed a bipolar construction whichprovided a six volt battery. The battery discharged across a K load andthe voltage observed with time is shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Li/MnO.sub.2 Battery Discharge                                                       Time    Voltage                                                               (hours) (volts)                                                        ______________________________________                                                0 (CCV)                                                                              6.56                                                                   1      5.983                                                                  5      5.584                                                                 10      5.501                                                                 20      5.504                                                                 30      5.492                                                                 40      5.468                                                                 50      5.355                                                                 60      5.128                                                                 70      3.823                                                                 80      3.027                                                          ______________________________________                                    

It is to be understood that modifications and changes to the preferredembodiment of the invention herein described can be made withoutdeparting from the spirit and scope of the invention. For example,bipolar batteries could be constructed of six or nine volts. Forexample, in FIG. 2 if two batteries were placed on top of each otherwith one of the current collectors removed, then a bipolar battery wouldbe constructed consisting of a current collector 18, anode 16, separator12, cathode 4, current collector 2, anode 16, separator 12, cathode 4and current collector 2.

What is claimed:
 1. A method for producing a cell comprising thesteps:(a) depositing an active cathode material containing at leastethylene carbonate and propylene carbonate onto a first conductivecurrent collector sheet and within an adhesive coated frame positionedat the peripheral area of the current collector sheet; (b) depositing onthe cathode material propylene carbonate or ethylene carbonate andpropylene carbonate and allowing said deposit to be absorbed into thecathode material (c) preparing a separator strip as follows:(a')dissolving ethylene carbonate in propylene carbonate to form a solution;(b') dissolving poly(ethylene oxide) in a solvent; (c') adding a metalsalt and a solvent along with the solution of step (a') to the dissolvedpoly(ethylene oxide) solution of step (b') to form a mix and mixing themixture to form a homogeneous mixture and drying said mixture to lessthan about 30 ppm water and then forming a separator strip from saidmixture; (d) placing the separator strip onto the cathode material, saidseparator strip being smaller than the area of the cathode collectorsheet and larger than the exposed area of the cathode material therebyleaving a portion of the adhesive coated frame area exposed; (e) placingan anode strip onto the separator and within the frame thereby leavingan exposed portion of the area of ht adhesive coated frame; (f) placinga second conductive current collector substantially the size of thefirst conductive current collector onto the anode and exposed area ofthe adhesive coated frame; and (g) heating at least the frame of thecell assembly at a pressure and temperature for a period of timesufficient to secure the first current collector to the adhesive coatedframe on one side and the opposite side of the adhesive coated frame tothe second current collector thereby producing a sealed cell.
 2. Themethod of claim 1 wherein in step (a) the active cathode material isdeposited onto the first current collector sheet and then the adhesivecoated frame is positioned around said active cathode material and ontothe peripheral area of the first current collector sheet.
 3. The methodof claim 1 wherein said first current collector and said second currentcollector is selected from the group consisting of copper, nickel andstainless steel.
 4. The method of claim 3 wherein said first currentcollector and said second current collector are copper.
 5. The method ofclaim 1 wherein said cathode material contains an active cathodematerial selected from the group consisting of manganese dioxide, ironsulfides, copper sulfides, silver chromates, lead oxides, bismuthoxides, copper oxides, nickel oxides, carbon monofluoride, vanadiumoxide, silver vanadium chromate and cadmium.
 6. The method of claim 5wherein the active cathode material is manganese dioxide.
 7. The methodof claim 1 wherein the homogenous mixture is deposited onto a releasablesheet and dried to less than about 30 ppm water.
 8. The method of claim7 wherein the solvent in step (b') is isopropyl alcohol.
 9. The methodof claim 7 wherein the solvent is step (c') is acetonitfile.
 10. Themethod of claim 7 wherein the separator film contains less than about 20ppm water.
 11. The method of claim 6 wherein the cathode materialcontains less than 30 ppm water
 12. The method of claim 1 wherein instep (f) the cell assembly is heated at a temperature . between 170° C.to 175° C. at a pressure of between 75 psig to 80 psig to seal the firstcurrent collector and the second current collector to the adhesive framethereby producing a sealed cell.
 13. The method of claim 1 wherein thecell assembly is encased in a plastic film.
 14. The method of claim 1wherein the cathode material comprises manganese dioxide and carbon andthe anode strip is lithium.
 15. The method of claim 1 wherein thecathode material comprises vanadium oxide; and the anode strip islithium.